The present invention relates to a method of conditioning marine seismic data obtained by a moving marine seismic receiver co-operating with a moving marine seismic source. Such a method is particularly effective with an "encoded" source, such as a marine vibrator, and at least one hydrophone, for instance a plurality of hydrophones or groups of hydrophones in a streamer, as the receiver, all of which are towed behind a vessel. An "encoded" source is a source which produces seismic energy over a prolonged period, for instance several seconds or tens of seconds, for each "shot" as opposed to an impulsive source, such as an array of air guns, which produces a burst of seismic energy over a short period, for instance several milliseconds or tens of milliseconds.
Marine seismic data acquisition has been performed using a marine vibrator source of the type comprising a hemispherical casing closed by a circular membrane which is caused to vibrate so as to act as a source of acoustic signals. The vibrator is deployed from flotation members which are towed behind a vessel, and is caused to vibrate by internal drive means controlled from the vessel so as to emit acoustic energy which travels through the water as propagating wave fronts. Suitable pressure transducers such as hydrophones, for instance in the form of a streamer, are likewise deployed from flotation members and towed behind the vessel so as to pick up acoustic signals reflected from the seabed and substrata.
The vibrator is energised or controlled by a signal whose frequency varies with time, for instance in accordance with a sweep function which causes the frequency to sweep from a low frequency to a high frequency for each "shot" recording. During data acquisition, the vessel moves at a constant relatively slow speed, generally of a few knots, so that the vibrator and the hydrophones are not stationary with respect to the earth, or reflecting body, but are continuously moving. The sweep function and the speed of movement relative to the earth are generally such that the vibrator and the hydrophones have moved a significant distance between the start and end of the sweep function.
U.S. Pat. No. 4,780,856 discloses a method of this type and suggests a suitable sweep function.
Data acquired by this type of method were previously processed using the same techniques as have been used for land-based vibrator seismic exploration, in which the source and the receivers are fixed. This resulted in a phase distortion of the wavelet in the final imaged section, the severity of which distortion depends on the angle of inclination or "dip" of the reflecting layer. In particular, if the reflecting layer has zero dip, there is no phase distortion whereas the distortion increases with the angle of dip.
These problems were recognized in a paper entitled "Marine Vibrators and the Doppler Effect" by W. H. Dragoset presented at the 50th Annual Meeting of the European Association of Exploration Geophysicists in 1988, and a possible solution to this problem was proposed and is disclosed in U.S. Pat. No. 4,809,235. The problem was analyzed in terms of the Doppler effect and it was proposed that a correction filter be applied during processing of the data. During such processing, the transmitted and received signals are correlated, and it was proposed that the filter should be applied after this correlation. This provides only an approximate solution to the problem and requires processing of the acquired data, which processing is dependent on the sweep function used to sweep the output frequency of the vibrator.